2 synchronized trigger, 9 guard channel option – Rainbow Electronics AT42QT1110-AZ User Manual
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9570H–AT42–02/10
AT42QT1110-MZ/AT42QT1110-AZ
4.8.2
Synchronized Trigger
In 11-key mode, if a time trigger is not enabled, the QT1110 operates in “synchronized” mode. In
this mode, SNS10K is used as a SYNC pin to trigger key acquisition, rather than using the
device’s internal clock. In this case the maximum number of keys is reduced to 10.
The SYNC pin can use one of two methods to trigger key measurements, selectable via bit 4 of
the Device Mode setup byte (see
): Low Level and Rising Edge.
With the Low Level method the QT1110 operates in “free run” mode for as long as the SYNC pin
is read as a logical “0”. When the SYNC pin goes high, the current measurement cycle will be
finished and no more key measurements will be taken until the SYNC pin goes low again.
The low level trigger should be a minimum of 1 ms so that there is sufficient time for the device
to detect the low level.
With the Rising Edge method all enabled keys are measured once when a rising edge is
detected on the SYNC pin. This allows key measurements to be synchronized to an external
event or condition.
For example, the SYNC pin can be used by the host to synchronize several devices to each
other. This would ensure that only one of the devices outputs pulses at any given time and
signals from one QT1110 do not interfere with the measurements from another.
Another use for synchronizing to the rising edge is to steady the signals when the device is
running off a mains transformer with insufficient mains frequency filtering that is causing a 50Hz
or 60Hz ripple on Vdd. If the mains voltage is scaled down with a simple voltage divider and
connected to the SYNC pin, then the key measurement can be triggered by the rising edge
detected at a positive going zero-crossing. Note that in this case, each key signal will be taken at
the same point in the cycle, so Vdd will be the same at each measurement for a given key and
the signals will be steadier.
4.9
Guard Channel Option
The device has a guard channel option (available in all key modes), which allows one key to be
configured as a guard channel to help prevent false detection (see
).
Guard channel keys should be more sensitive than the other keys (physically bigger or larger
Cs), subject to burst length limitations (see
).
With guard channel enabled, the designated key is connected to a sensor pad which detects the
presence of touch and overrides any output from the other keys using the chip’s AKS feature.
The guard channel option is enabled by the Guard Key setup byte (see
).
With the guard channel not enabled, all the keys work normally.
Note:
If a key is already “in detect” when the guard channel becomes active, that key will
remain in detect and the guard key will not activate until the active key goes out of
detect.